The present invention relates generally to solutions for contactless reading of data. More particularly the invention relates to an apparatus according to the preamble of claim 1 and a method according to the preamble of claim 10. The invention also relates to a computer program product according to claim 17 and a computer readable medium according to claim 18.
Traditionally, data from transponder tags have been registered via transmitter-receiver equipment, where the signal from the tag is demodulated based on envelope detection, i.e. by an approach involving amplitude demodulation. Although, the components required thereby can be made relatively uncomplicated, the technology is fairly inefficient in terms of power consumption.
U.S. Pat. No. 4,260,990 discloses an antenna system having a transmitting antenna with at least one loop lying in a plane, and a receiving antenna having at least two twisted loops lying in a common plane with each loop being twisted 180 degrees and in phase opposition with each adjacent loop. The transmitting and receiving antennas are disposed in spaced substantially parallel relationship across an aisle or passage through which a resonant tag circuit must pass for detection. Hence, the transmitter and receiver are physically separated from one another by a distance given by the width of the aisle/passage for passing the resonant tag circuit through the emitted radio field.
EP 608 961 and EP 646 984 show examples of electromagnetic detection systems for detecting or identifying labels containing at least one resonant circuit, where a label's resonance frequency is indicative of its identity.
WO 94/19781 describes a system for identifying an electronic transponder, where the system includes a transmitter unit and at least one transmitting antenna coupled thereto for generating an electromagnetic interrogation field. A detection unit in the system detects signals emitted by the transponders when they are located in the interrogation field. To this aim, the detection unit has a means for detecting signals coming from different transponders on the basis of strength differences between these signals. In one embodiment, in order to tune the receiver unit to frequency band of the interrogation field emitted by the transmitter unit, a signal representing the frequency or frequencies of the interrogation field is supplied from the transmitter to the receiver unit through an interconnecting wire line.